Let's Encrypt Installation on Plesk:

1. Log in to Plesk Control Panel: Open your browser and log in to the Plesk control panel.

2. Select Your Website: Choose "Websites" from the left menu and then select the domain for which you want to install the Let's Encrypt certificate.

3. SSL/TLS Certificates: On the page of the selected domain, navigate to the "SSL/TLS Certificates" tab.

4. Switch to Let's Encrypt Tab: In the "SSL/TLS Certificates" page, find the "Let's Encrypt" tab.

5. Add a Certificate: In the "Let's Encrypt" tab, click on the "Add Certificate" or a similar button.

6. Enter Required Information: In the opened window, fill in the required information, such as domain name, www prefix option, email address, etc. Then click on the "Get Certificate" or a similar button.

7. Certificate Installation: Plesk will communicate with Let's Encrypt and generate the certificate. This process may take a few minutes. Once completed, you will see a success message.

8. Certificate Settings: After installing the certificate, go back to the "SSL/TLS Certificates" page and select the newly created certificate. You can make any necessary settings.

9. Restart the Web Server: To activate the certificate, restart your web server using the "Restart Web Server" or a similar option in your Plesk control panel.

Your website should now be secure with an SSL/TLS certificate provided by Let's Encrypt. The steps might vary slightly depending on the interface and version of your Plesk, but generally, you will follow similar steps.